docs: def get_constantvalue(self):
          """
          the constant pool index for this field, or None if this is not a
          contant field
  
          reference: http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.2
          """  # noqa
  
          buff = self.get_attribute("ConstantValue")
          if buff is None:
              return None
  
          with unpack(buff) as up:
              (cval_ref, ) = up.unpack_struct(_H)
  
          return cval_ref